Detailed explanation of the principle of svpwm variable frequency speed regulation The difference between svpwm and SPWM

This article is mainly about the related introduction of svpwm frequency conversion speed regulation, and focuses on the detailed introduction of svpwm and SPWM.

SVPWM

The main idea of ​​SVPWM is to use the three-phase symmetrical sine wave voltage power supply as the reference standard for the ideal flux circle of the three-phase symmetrical motor stator, and appropriately switch the different switching modes of the three-phase inverter to form a PWM wave. The actual flux linkage vector is used to track its accurate flux linkage circle. The traditional SPWM method starts from the perspective of the power supply to generate a sine wave power supply with adjustable frequency and voltage, while the SVPWM method considers the inverter system and the asynchronous motor as a whole, the model is relatively simple, and it is also convenient for the microprocessor to realize real-time control.

principle

An ordinary three-phase full bridge is three half bridges composed of six switching devices. The combination of these six switching devices (the signals of the upper and lower half bridges of the same bridge arm are opposite) have a total of 8 safe switching states. Among them, 000 and 111 (here represent the switching states of the three upper bridge arms) these two switching states will not generate effective current in the motor drive. Therefore, it is called a zero vector. The other 6 switch states are six effective vectors respectively. They divide the 360-degree voltage space into a 60-degree sector for a total of six sectors. Using these six basic effective vectors and two zero quantities, any vector within 360 degrees can be synthesized.

When you want to synthesize a certain vector, first decompose this vector into the two closest basic vectors, and then use these two basic vectors to express, and the effect size of each basic vector is represented by the length of the effect. The voltage vector is used to synthesize the required voltage vector according to different time ratios. This ensures that the generated voltage waveform is similar to a sine wave.

When the variable frequency motor is driven, the vector direction changes continuously, so we need to constantly calculate the vector action time. For the convenience of computer processing, timer calculation is generally used during synthesis (for example, once every 0.1ms). In this way, we only need to calculate the time for the two basic vectors to act in 0.1ms. Since the calculated sum of the two times may not be 0.1ms (less than this), the remaining time will be inserted into the appropriate zero vector according to the situation. Because in this processing, the synthesized drive waveform is very similar to PWM. So we also call it PWM, and because this kind of PWM is synthesized based on the voltage space vector, it is called SVPWM.

The principle of svpwm frequency conversion speed regulation

SVPWM principle

The starting point of voltage space vector PWM (SVPWM) is different from that of SPWM. SPWM modulation starts from a three-phase AC power supply and focuses on how to generate a three-phase symmetrical sinusoidal power supply that can be adjusted with voltage and frequency. The SVPWM treats the inverter and the motor as a whole, synthesizes the desired voltage vector with eight basic voltage vectors, establishes the switching state of the inverter power device, and based on the relationship between the motor flux and voltage, so as to realize the control of the motor. Constant flux variable voltage frequency conversion speed regulation. If the stator resistance voltage drop is neglected, when an ideal sinusoidal voltage is applied to the stator winding, since the voltage space vector is a rotation vector of equal amplitude, the air gap magnetic flux rotates at a constant angular velocity and the trajectory is circular.

The voltage utilization rate of SVPWM is 15% higher than that of SPWM. This is the biggest difference between the two, but the two are not isolated modulation methods. Typical SVPWM is a rule that adds zero sequence component to the phase modulation wave of SPWM. The result of sampling, so SVPWM has a corresponding SPWM form. Conversely, some SPWM methods with superior performance can also find corresponding SVPWM algorithms, so the two are consistent in the general direction of harmonics, but SPWM is easy to implement by hardware circuits, and SVPWM is more suitable for digital control systems.

In Figure 1, the switching vector [abc]T has 8 values, that is, there are 8 combinations of the switching states of the 6 IGBTs. These 8 switching combinations determine 8 basic space vectors, as shown in Figure 3. The voltage Uout surrounded by two adjacent basic space vectors U0 and U60 is mapped to the sum axis 6-8, and formula 1 is obtained, where T represents the length of a PWM cycle, and T1 and T2 are in a cycle time T, respectively The basic space vectors U0 and U60 are the respective action time, T is the action time of the zero vector in one cycle, T0+T1+T2=T.

The principle of svpwm variable frequency speed regulation explains the difference between svpwm and SPWM in detail

If formula 2 is defined, the action time T1 and T29-10} of the two basic vectors surrounding this sector in each sector can be obtained in one PWM cycle, as shown in Table 1.

The principle of svpwm variable frequency speed regulation explains the difference between svpwm and SPWM in detail

For formula 3, define 3 variables a, b, c, if Vref1》0, then a=1, otherwise a=0; if Vref2》0, then b=1, otherwise b=0; if Vref3》0, then c=1, otherwise c=0. Set N=4c+2b+a, then it is easy to get the corresponding relationship between N and the number of sectors, as shown in Table 1.

The principle of svpwm variable frequency speed regulation explains the difference between svpwm and SPWM in detail

In order to ensure the duty cycle of the three-phase bridge arm in a PWM cycle, the comparison values ​​that should be set are defined as Tcm1, Tcm2 and Tcm3 respectively, and formula 4 is defined, then N and the number of sectors and Tcm 1, Tcm2 The relationship with Tcm3 is shown in Table 1. Compare Tcm1, Tcm2, and Tcm3 with the DSP chip timer set in continuous increase/decrease mode to obtain PWM pulses, control the on and off of the three bridge arms in Figure 1, so as to generate phases in the 3-phase stator windings of the PM SM A sine wave current with a difference of 120° forms a circular magnetic field to drive the motor.

The difference between svpwm and SPWM

According to the principle of equal waveform area, the area of ​​each rectangular wave is equal to the area of ​​the sine wave at the corresponding position, so this sequence of rectangular waves is equivalent to the expected sine wave. This modulation method is called sine wave pulse width modulation (SPWM), and this sequence of rectangular waves is called SPWM wave.

The principle of svpwm variable frequency speed regulation explains the difference between svpwm and SPWM in detail

The principle of svpwm variable frequency speed regulation explains the difference between svpwm and SPWM in detail

The picture shows the three-phase PWM waveform, where

urU, urV, urW are U, V, W three-phase sinusoidal modulation wave uc is a bipolar triangular carrier;

uUN', uVN', uWN' are the rectangular waveforms of the phase voltage between the U, V, W three-phase output and the neutral point N of the power supply;

uUV is the rectangular waveform of the output line voltage, and its pulse amplitude is +Ud and -Ud;

uUN is the phase voltage between the three-phase output and the neutral point N of the motor.

The classic SPWM control mainly focuses on making the output voltage of the transformer inverter as close to the sine wave as possible, and does not take into account the waveform of the output current. The current hysteresis tracking control directly controls the output current to change near the sine wave, which is one step ahead of requiring only the sine voltage. However, the ultimate goal of AC motors requiring input of three-phase sinusoidal current is to form a circular rotating magnetic field in the motor space, thereby generating a constant electromagnetic torque.

If you aim at this goal, regard the inverter and the AC motor as a whole, and control the work of the inverter in accordance with the tracking of the circular rotating magnetic field, the effect should be better. This control method is called "flux tracking control", and the trajectory of the flux linkage is obtained by using different voltage space vectors alternately, so it is also called "voltage space vector PWM (SVPWM, Space Vector PWM) control".

The principle of svpwm variable frequency speed regulation explains the difference between svpwm and SPWM in detail

The principle of svpwm variable frequency speed regulation explains the difference between svpwm and SPWM in detail

With the switching of the inverter's working state, the amplitude of the voltage space vector does not change, and the phase rotates by π/3 each time until the end of a cycle. In this way, the 6 voltage space vectors have rotated 2π radians in one cycle, forming a closed regular hexagon.

In one cycle, the six flux linkage space vectors are radial, the tails of the vectors are all at point O, and the trajectory at the top is a regular hexagon surrounded by the six voltage space vectors.

The principle of svpwm variable frequency speed regulation explains the difference between svpwm and SPWM in detail

At any moment, the direction of the generated flux linkage increment is determined by the applied voltage, and its amplitude is proportional to the time of applying the voltage. If the AC motor is only powered by a conventional six-beat step-wave inverter, the flux trajectory is a hexagonal rotating magnetic field, which is obviously not like the circular rotating magnetic field generated when a sine wave is supplied, which can make the motor obtain a uniform speed. run. If you want to approximate a circle, you can increase the number of switching times. It is assumed that the flux linkage increment is composed of the 4 segments 11, 12, 13, 14 in the figure. At this time, the phase of the voltage space vector applied in each segment is different, which can be obtained by the method of linear combination of the basic voltage vectors.

The traditional SPWM method starts from the point of view of the power supply and aims to generate a sine wave power supply with adjustable frequency and voltage. The SVPWM method considers the inverter system and the asynchronous motor as a whole, the model is relatively simple, and it is also convenient for real-time control of the microprocessor. The generation principle of SVPWM itself has nothing to do with PWM, just like it. The driving waveform synthesized by SVPWM is very similar to PWM, so we also call it PWM, and because this kind of PWM is synthesized based on voltage space vector, it is called it. SVPWM too.

In summary, the principles and sources of SVPWM and SPWM are very different, but they do achieve the same goal by different means. SPWM is modulated by triangle wave and sine wave, while SVPWM can be regarded as modulated by triangle wave and sine fundamental wave with a certain third harmonic content. In contrast, SVPWM has the following characteristics:

(1) Although there are multiple switches between each cell, each switch involves only one device, so the switching loss is small.

(2) The three-phase PWM wave is directly generated by using the voltage space vector, and the calculation is simple.

(3) The maximum value of the fundamental wave of the inverter output line voltage is the DC side voltage, which is 15% higher than the output voltage of the general SPWM inverter

Conclusion

That's it for the related introduction of svpwm frequency conversion speed regulation. I hope this article can be helpful to you.

Related reading recommendation: SVPWM principle introduction and feature summary Related reading recommendation: svpwm algorithm principle and analysis

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